Carlson in U.S. Pat. No. 4,064,521, incorporated herein by reference, has disclosed a photocell incorporating amorphous silicon fabricated in a glow discharge in a silane gas as the light sensitive material. The photocell typically consists of a comparatively thick, about 500 nanometers (nm), layer of intrinsic amorphous silicon bounded by thin, opposed p-type and n-type layers, each about 10 nm thick, with the light entering the device through either the p-type or n-type layer. The p-type and n-type layers are highly absorbing and carriers generated therein typically recombine before they reach the depletion region of the device to be collected as current. There is a limit to how thin these layers can be made to avoid this problem since, if the layer is too thin, there is insufficient space charge available to fully deplete the intrinsic region. Thus the optimum thickness of the p- and n-type layers are typically chosen as a compromise between low absorption and sufficient space charge.
Pankove, in U.S. Pat. No. 4,109,271, incorporated herein by reference, has disclosed that silicon deposited in an atmosphere which includes carbon forms an amorphous silicon-carbon alloy which has a wider bandgap, and thus less absorption, than amorphous silicon. A suitably doped amorphous silicon-carbon alloy is substituted for either the p-type or n-type layer through which light enters the photocell, thereby reducing the magnitude of the absorption in this layer. However, the thickness of the silicon-carbon alloy is limited by the transport of majority carriers through this layer to an electrode since the transport appears to occur through gap states rather than the band states. The optimum thickness of the silicon carbon alloy layer is then also a compromise between a thin layer sufficient for good charge transport and a thick layer which provides sufficient space charge to fully deplete the intrinsic region. It would thus be desirable to have a photocell structure which can make use of a material such as a silicon-carbon alloy having the desired wide bandgap properties but which will also have good majority carrier transport properties and sufficient space charge to fully deplete the intrinsic region of the device.